PROJECT SUMMARY This project is in response to NIMH RFA-MH-19-120 calling for the development and validation of new screening methods for autism spectrum disorder (ASD) that can be used in infancy (0-12 months of age). We will deploy a cost-effective, high-throughput methodology utilizing performance-based, objective, and highly quantitative eye- tracking assays of social visual engagement?the way in which infants visually explore, engage, and ultimately learn from and adapt to their surrounding world?collected on a standalone, mobile eye-tracking data collection device, created in our lab (the Marcus Autism Center Investigational Device, MAC-ID). We will collect eye- tracking data from a population-based sample of N=2,000 9-month-old infants recruited consecutively in primary care practices at the time of their 9-month well-child visits. These infants will be followed longitudinally from 9 until 26 months for completion of a series of sequential screening and clinical ascertainment procedures designed to maximize sensitivity and achieve clinician-best-estimate diagnostic assignments of ASD vs. non- ASD and Affected [including ASD and non-ASD developmental delays] vs. Unaffected. We will measure the accuracy of eye-tracking-based screening at the age of 9 months relative to clinician best estimate diagnosis at 24 months (primary analysis), and we will measure dimensional agreement between eye-tracking assays at 9 months relative to outcome levels of social disability, verbal ability, and nonverbal cognitive ability (secondary analyses). Our overarching goal is to develop high-quality, objective, performance-based tools that can function as an effective and community-viable means of screening for ASD and other actionable developmental delays in infancy, to ultimately facilitate improved access to and benefit from early intervention.